29th International Cosmic Ray Conference Pune (2005) 5, 335–338

Tests of PMT uniformity for the GRAPES-3 experiment
A. Oshima , S.K. Gupta , Y. Hayashi , N. Ito , A. Jain , S. Karthikeyan ,S. Kawakami , H. Kojima , T. Matsuyama , P.K Mohanty , T. Nonaka ,K.C. Ravindran , H. Tanaka , S.C. Tonwar and T. Yoshikoshi






(a) Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India(b) Graduate School of Science, Osaka City University, Osaka 558-8585, Japan

Presenter: T. Nonaka (oshima@alpha.sci.osaka-cu.ac.jp), jap-oshima-A-abs1-og27-poster
It is observed that the response of the PMT varies across the photo-cathode leading to certain non-uniformity.
To select PMTs having uniform efficiency across the photo-cathode, an X-Y scanner with appropriate lightsource has been designed to measure PMT photo-cathode response. Here we present the results of uniformity
measurements on the PMT response.

1.

Introduction

GRAPES-3 is an extensive air shower experiment operating at Ooty (E76.7 , N11.4 ,2200m asl), in south
India. It consists of about 300 scintillation detectors each having an area of 1m and 16 large area muon
detectors having a total area of 560m . There are two kinds of scintillation detectors. The first type is conicalshaped detector with a PMT directly viewing the scintillator. The second type is flat detectors overlaid withwave-length shifting fibers in parallel grooves called fiber detector. In the case of conical shaped detector, nonuniformity of quantum efficiency is not of any significance because the distance between PMT and scintillator
is large and the photons from scintillator reach PMT surface after several reflections and uniformly illuminate
the PMT photo-cathode. On the other hand, in the flat type detector the photons collected by the WLS fibers aretransferred to the PMT through a optically coupled cookie. Because the WLS fibers are coupled to a local areaof the PMT surface, the response of the PMT varies with the position of the cookie. Thus the response of thePMT becomes non-uniform if the quantum efficiency varies across the face of the PMT. A flat detector has 4
scintillator blocks each viewed by a group of 18 WLS fibers and each group is terminated at a separate cookie.
Since these 4 cookies are attached to the PMT surface at a different location, non-uniformity of quantum
efficiency becomes important for the flat type detector. We are using a fast 2 inch diameter photomultiplier,
ETL 9807B with blue-green sensitive bialkali photo-cathode from Electron Tubes [1]. To select PMTs having
uniform quantum efficiency, we have designed a system for measurement of the uniformity using a precision
X-Y scanner.
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2.

SystemIn figure 1 the system used to measure the PMT uniformity is shown. It contains a light tight box where theX-Y scanner and nine sockets facing the same direction, to mount a set of nine PMTs are placed. Thus a totalof nine PMTs can be scanned simultaneously. The main component of this system is the X-Y scanner, which isoperated using two stepping motors to move it along two orthogonal directions (X and Y) with high positional
resolution, in steps of 1 m. There are two plates, placed on the X-Y scanner each having 9 pin-holes of 1mm diameter to serve as a nine beam collimator . Under the lower plate there is another plate on which ninegreen LEDs are fixed as light source. The wave-length of the LED output is 525 nm which is in the range ofsensitivity of the emission spectra of WLS fiber[2]. When triggered, the LEDs produce a light pulse, with awidth of 30 ns. The light pulse emitted from LED is collimated through the two pin holes of the two plates336

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above it, finally reaching the PMT surface with small angular spread. We have used a CAMAC based dataacquisition system for generating the trigger pulse for the LED. The operation of X-Y scanner is controlled bya serial interface.
Trigger
ADC generator

CAMAC

personal computer

PMTLEDY−axisX−axsisX−Y stagecontroller

X−Y stage

Figure 1. System for PMT uniformity test. A charge sensitive 32 channel ADC module and a trigger generator module islocated in CAMAC crate. Serial communication is used between PC and X-Y controller to position the X-Y scanner.

3.

Experiment

The X-Y scanner is used in a programmed mode to scan a square area larger than the size of the PMT photocathode by scanning the PMT on pre-determined step size. At each location data is collected by triggering theLED a fixed number of times as detailed below, which takes about one hour to complete scan.
1. Step width in X and Y directions is 1 mm.
2. Intensity of LED is adjusted to yield around 100 photo-electron.
3. Number of times LED triggered at each position is 200.
4. Scanning area is 65

65 mm to cover a total area of PMT surface.
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Tests of PMT uniformity ...

4.

Results

In figure 2 uniformity of the PMT photo-cathode is shown for a typical PMT. Vertical axis represents mean
ADC counts after subtracting the pedestal. To characterize the variation of the quantum efficiency of the PMTsurface, we determined the mean and rms deviation within the area of a circle of radius 20 mm from the centerof the PMT as shown by an arrow in figure 3. This is necessary because the PMTs display a rapid decrease in
their quantum efficiency near the outer edge. The present PMT shows an rms variation of 6.5%. within 20 mmof the center. In figure 3 the response of the same PMT is shown along two different directions. The plot on
the left shows the PMT response along X-axis and the plot on the right shows the response along Y-axis. In thefigures 2 and 3 one can see a dip in the response and the depth of the dip is about 30 % below the mean value.
Almost all PMTs we checked displayed such a dip in their response at the same location. If the area of the dip
is excluded then the rms variation of this PMT reduces to 4.4% which may be considered to be fairly uniform.
In figure 4 the response of a PMT having large non-uniformity is shown. This PMT has an rms variation of10.7% in the selected fiducial area.
PMT0: AVE=968 :STD=63

1400
1200
1000
800
600
400
2000010203040x506070 010203040506070

yFigure 2. Uniformity of a typical PMT, the rms variation of this PMT is about 6.5%.

1400x=30 mm

1200

1200

1000

1000

ADC counts

ADC counts

1400

800
600
400
2000-10y=30 mm

800600400
2000102030mm

405060700-10010203040506070mm

Figure 3. Left plot shows the uniformity along the X-axis and the one on the right shows response along Y-axis. Arrows
show the range of an area used to determine the mean and rms variation.

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1400

x=30 mm

1200

1200

1000

1000

ADC counts

ADC counts

1400

800
600
400
2000-10y=30 mm

800
600
400
200

0102030405060mm

700-10010203040506070mm

Figure 4. A PMT having large variation in its response. In the right plot response varies from -30% to 40%. from its meanvalue.

5.

Conclusion

The X-Y scanner developed to measure the response of the PMT has successfully measured the uniformity ofquantum efficiency across the photo-cathode. The X-Y scanner allows identification of the area of uniform
response on the PMT surface which can be used to locate the WLS fiber cookies. This has permitted successful
utilization of a number of PMTs with rather non-uniform response which otherwise would not have beenpossible.

6.

Acknowledgements

We thank K. Manjunath, K. Ramadass, G.P. Francis for help in assembling the X-Y scanner and for conducting
various tests.

References[1] DS 9807B Series Issue, Electron Tubes Inc.
[2] Scintillation Materials, Kuraray Co.,Ltd.